The present invention relates to high frequency induction logging systems, and more particularly, to a method for using a high frequency induction logging device (HFIL) to locate the position of an interface between two different zones having different dielectric constants even if two zones have the same resistivity. For example, one zone may be a fresh water zone while the second zone is a hydrocarbon zone. The invention is also useful in determining the overall thickness of a hydrocarbon bearing zone which is bounded on the top and bottom by zones having different dielectric constants even if all three zones have the same resistivity.
In U.S. Pat. No. 4,278,941, which is incorporated in the present specification for background purposes, there is described a high frequency induction logging method and apparatus utilizing a transmitter and at least two receivers spaced to one side of the transmitter. The voltage signals induced in the two receivers are measured to determine the attenuation and phase shift between the two receivers. From the phase shift and attenuation one can derive the dielectric constant and resistivity of the formation. The patent also describes the change in the phase difference as the tool passes an interface. As shown in the patent, the accurate determination of the location of the interface is difficult if not impossible without knowledge of both the dielectric constant and resistivity of the formations.
U.S. Pat. Nos. 4,185,238 and 4,209,747 describe high frequency induction logging devices in which two receiver coils are used at each of two separate locations. One pair of receiver coils is used to determine the attenuation nearer to the transmitter while the second pair of receiver coils is used to determine the phase shift at a position farther removed from the transmitter. The patents describe the effect of the borehole on the induced current and how the receiver coils respond to different parts of the formation if one was measuring the difference in amplitude as contrasted to the difference in phase. The amplitude of the signal is affected more by the far or remote portions of the formation surrounding the borehole while the phase is affected more by the near or adjacent portions of formations. Thus, in order to provide a logging tool that will accurately measure both the difference in amplitude and phase shift the patent suggests that the difference in amplitude be measured at a near set of receiver coils while the difference in phase be measured at a far or remote set of receiver coils. From the above, it can be seen that while the two patents provide a more accurate measurement of the dielectric constant of the formations, they do not provide a more accurate definition of the interface nor the thicknesses of the various formations. The two patents are the same as U.S. Pat. No. 4,278,941 in that the receiving coils are all positioned to one side of the transmitter.